To complete a well, one or more formation zones adjacent a wellbore are perforated to allow fluid from the formation zones to flow into the well for production to the surface or to allow injection fluids to be applied into the formation zones. A perforating gun string may be lowered into the well and the guns fired to create openings in a casing and to extend perforation tunnels into the surrounding formation.
The explosive nature of the formation of perforation tunnels shatters sand grains of the formation. A layer of “shock damaged region” having a permeability lower than that of the virgin formation matrix may be formed around each perforation tunnel. The process may also generate a tunnel full of rock debris mixed in with the perforator charge debris. The extent of the damage, and the amount of loose debris in the tunnel, may be dictated by a variety of factors including formation properties, explosive charge properties, pressure conditions, fluid properties, and so forth. The shock damaged region and loose debris in the perforation tunnels may impair the productivity of production wells or the injectivity of injector wells.
One known method of achieving removal of debris from the perforation tunnels formed in the surrounding formation involves positioning a standard perforating gun or closed tube provided internally with a detonating cord and shaped charges of limited energy within a wellbore adjacent existing tunnels. Pressure within the wellbore is higher than the substantially lower atmospheric pressure inside the closed tube. With this arrangement, explosion of the charges inside the tube will cause openings to be formed in the tube only and not the casing such that a dynamic underbalance pressure condition or pressure differential is created between the wellbore and the inside of the tube. The underbalanced pressure condition results in a suction force that will draw debris out of the perforation tunnels formed in the surrounding formation into the tube enabling the well to flow more effectively. After a surge of debris from the perforation tunnels, the filled tube is removed from the wellbore and disposed of.